To give you the idea of what an electron arrangement is, here is an example: 

 The dots on the diagram represent electrons and each of the circles represents an electron energy level. To work out the electron arrangement from the diagram we start at the circle closet to the centre and work out noting the number of electrons in each energy level giving 2,8,8. This is the electron arrangement of argon. However if argon was forced to loose an electron it would have an electron arrangement of 2,8,7. If it was forced to gain an electron the arrangement would be 2,8,8,1 – there can only be eight outer electrons in each energy level so it adds another level.